This invention relates to time-interval conversion circuits, particularly time-interval conversion circuits that convert a time interval between edge transitions of two signals into a third signal representative of that interval.
In electronic systems and instruments it is often desirable to employ a circuit that converts the time interval between two signals into analog voltage or digital information. For example, such conversion circuits may be used in oscilloscopes to reveal, in real time, modulation and other timing dynamics between signals. Typically, very high conversion rates and very high resolution are sought from such conversion circuits. For example, when used in oscilloscopes, such conversion circuits may need to achieve conversion rates and resolution of, respectively, greater than 100 MHz and less than 50 picoseconds.
Time intervals have been measured using circuits that can be referred to collectively as time interpolators. In their simplest form, time interpolators measure by counting clock cycles that are internally generated by the time interpolator during the time interval being measured. Although resolution is improved by using a high frequency clock, resolution is subject to an ambiguity of plus or minus one clock period. That ambiguity is introduced by the absence of coincidence between the clock and the edge transitions describing the time interval being measured. For example, if the edge transitions occur in the middle of the high state of the first and last clock cycle, the time measurement will reflect an excess clock period.
To correct for the ambiguity, time interpolators must measure the interval between the edge transitions of the signals and the clock. In linear interpolation, for example, the edge transition that starts the time interval charges a capacitor until the clock makes a low to high transition, at which time the charged capacitor is discharged at a known rate that is a small fraction of the charging rate. The discharge interval is measured by separately counting clock cycles for that interval. The time between the starting edge transition and the clock's rising edge equals (i) the product of the number of clock cycles counted for discharge, multiplied by the clock period, multiplied by the discharge rate, and (ii) divided by the charge rate. This measurement by interpolation and an analogous measurement made for the edge transition that ends the time interval are added to, or subtracted from, the time interval measurement first described above to obtain the overall measurement.
With linear interpolation, additional resolution is achieved at the cost of introducing undesirable delays associated with the discharge of a capacitor. In addition, the accuracy of the linear interpolator is limited by the frequency and accuracy of the clock, as well as by the performance of the counters and the charging and discharging circuitry. Moreover, the linear interpolator delivers a count of clock cycles that requires digital-to-analog conversion, which is generally accomplished using techniques that are not real time and are slow.
Time intervals have also been measured using a circuit that has been referred to as a time trap. A time trap circuit measures time intervals by using a delay line of known propagation rate. The delay line is tapped at equally spaced intervals along its length so that, as a first input signal propagates along the delay line, the successive taps provide a means to detect the signal's progress. Each tap corresponds to a quantum of time equal to the total propagation time of the delay line divided by the total number of taps. Each tap connects to an input of a memory device such as a digital latch so that, when the memory device is strobed by a second input signal, the latch will capture a pattern of digital information that represents the time interval between the first propagating input signal and the strobe signal.
In the time trap circuit, resolution is determined by the number of taps along a given length of delay line: a large number of taps minimizes the spatial intervals between the taps and, thereby, minimizes the quantum of time corresponding to each tap. However, the number of taps is limited by the inherent losses of the delay line and the input capacitance of the latch, as well as by minimum spacing requirements of each tap.
Accordingly, there is a need for an improved technique and circuit for converting time intervals between edge transitions of two voltage signals into a voltage or digital representation thereof in real time, with high resolution and accuracy.